MAMMARY GLANDS. 379 



thick, white curd, and the other a thin, yellowish fluid. This 

 turning sour and ultimate curdling depends upon a change brought 

 about in one of its most important constituents, namely, milk- 

 sugar, by means of a process of fermentation. The milk-sugar, 

 in the presence of certain forms of bacteria, ferments, and gives 

 rise to lactic acid. When the quantity of lactic acid is suffi- 

 cient, it not only makes the milk sour, but also precipitates an- 

 other of its important constituents, namely, casein. This albu- 

 minous body in its coagulation entangles the fat of the milk, and 



FIG. 167. 



Section of Mammary Gland during active lactation (human). (a) Sac- 

 cules lined with regular epithelium. (6) Connective tissue between the 

 alveoli. (Cadiat.) 



we have thus formed the curd of cracked milk, whilst the whey 

 consists of the acid, salts, and remaining milk-sugar. 



Although the curdling of milk depends on the coagulation of 

 an albuminous body, it is never produced by boiling fresh milk, 

 because the chief proteid is casein, a form of derived albumin 

 (alkali-albumin), which does not coagulate by heat. 



When milk is preserved from impurities, and kept in a cool 

 place, a thick, yellow film soon collects on the top of the fluid ; 

 the thickness of this layer the cream may be taken as a rough 

 gauge of the richness of the milk, for milk consists of a fine 



